In the ancient past, said Michael Mumma, a senior scientist at NASA's Goddard Space Flight Center, Mars had an ocean that covered about 20 percent of the planet's surface area — "a respectable ocean." The body of water was about 5,000 feet (1,500 meters) deep, on average. Today, only 13 percent of that ancient ocean remains, locked in the polar ice caps.

One of the James Webb Space Telescope, soon to be the world’s premier orbiting infrared space observatory, first tasks will be to to learn why Mars lost so much water over its 4.5-billion-year history. Today, it is a frigid desert world with a carbon dioxide atmosphere 100 times thinner than Earth’s. But evidence suggests that in the early history of our solar system, Mars' surface likely hosted an ocean as deep as the Mediterranean Sea. As the planet's atmosphere thinned, however, most of the ocean was lost to space. The remainder of the water is locked in the Martian ice caps.

Hydrogen atoms escaped from the Mars upper atmosphere, while water containing heavy hydrogen (deuterium) remains trapped on the planet. The escape of hydrogen helped to turn Mars from a wet planet 4.5 billion years ago into a dry world today.

"Now we know that Mars water is much more enriched than terrestrial ocean water in the heavy form of water, the deuterated form," Mumma, said in a 2015 video. "Immediately that permits us to estimate the amount of water Mars has lost since it was young."

The ancient Martian ocean would have been saltier than the Great Salt Lake and nearly as salty as the Dead Sea, rendering it nearly inhospitable to life. "If there was any life on Mars, it would have needed to start off at high acidity and high salinity," said Nicholas Tosca, at St. Peters College, Oxford University. "Life on Mars would require biology that was completely different from any we know on Earth."

Mars will be targeted as part of a Guaranteed Time Observation (GTO) project led by Heidi Hammel, a planetary astronomer and executive vice president of the Association of Universities for Research in Astronomy (AURA) in Washington, D.C. The GTO program provides dedicated time to the scientists who have worked with NASA to craft the science capabilities of Webb throughout its development. Hammel was selected by NASA as a JWST Interdisciplinary Scientist in 2003. Mars will be visible to Webb from May to September 2020 during its first year of operations, known as Cycle 1.

Mars has been visited by more missions than any other planet in our solar system. It is currently orbited by six active spacecraft, while two rovers trundle across its surface. Webb offers several capabilities that complement these up-close missions.

One key asset is Webb’s ability to take a snapshot of the entire disk of Mars at once. Orbiters, in contrast, take time to make a full map and therefore can be affected by day-to-day variability, while rovers can only measure one location. Webb also benefits from excellent spectral resolution (the ability to measure small differences in wavelengths of light) and a lack of interfering atmosphere that plagues ground-based measurements from Earth.

That said, observing Mars with Webb will not be easy. “Webb is designed to be able to detect extremely faint and distant targets, but Mars is bright and close,” explained Geronimo Villanueva of NASA’s Goddard Space Flight Center, Mars lead on the GTO project. As a result, the observations will be carefully designed to avoid swamping Webb’s delicate instruments with light.

“Very importantly, observations of Mars will also test Webb’s capabilities in tracking moving objects across the sky, which is of key importance when investigating our solar system,” said Stefanie Milam at NASA’s Goddard Space Flight Center, Greenbelt, Md. who is coordinating the solar system program with Webb.

Much of the water Mars once held was lost over time due to ultraviolet light from the Sun breaking apart water molecules. Researchers can estimate how much water vanished by measuring the abundance of two slightly different forms of water in Mars’ atmosphere – normal water (H2O) and heavy water (HDO), in which one hydrogen atom is replaced by naturally occurring deuterium. The preferential escape of lighter hydrogen over time would then lead to a skewed ratio of H2O to HDO on Mars, indicative of how much water has escaped into space. Webb will be able to measure this ratio at different times, seasons and locations.

“With Webb, we can obtain a real and accurate measurement of the ratio of H2O to HDO across Mars, permitting us to determine how much water was truly lost. We also can determine how water is exchanged between polar ice, the atmosphere, and the soil,” said Villanueva.

Although most of the water on Mars is locked up in ice, the possibility remains that some liquid water could exist in underground aquifers. These potential reservoirs could even host life. This intriguing idea received a boost in 2003, when astronomers detected methane in the Martian atmosphere. Methane could be generated by bacteria, although it could also come from geological processes. Data from Webb could provide new clues to the origin of these methane plumes.